Thank you Boss for your answer,
RRDTOOL & snmp were not installed!
Thanks to you, I found the correct path was the standard path!
With a better sources.list, I finally could install PHP5 & Mysql5 today. I've been able to install Myprepaid 4.0 without error!
I am still having following error after adding an Access point
dns name: my_dyndns.com
ip adress : 192.168......
and clicking on its logo :
Fatal error: Call to undefined function snmp_set_valueretrieval()
in /var/www/phpmyprepaid0.4/www/include/ap/snmp_default.php on line 12
Thanks again for your answer.
Nice weekend to all.